<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* id选择器 */
        #box{
            height: 100px;
            width: 100px;
            background-color: green;
        }
        /* 类选择器 */
        .box{
            height: 150px;
            width: 150px;
            background-color: pink;
        }
        /* 
        span 行内元素,它需要内容撑起来,单独设置宽高不生效
        span的display是inline
        修改它的 */
        span{
            height: 200px;
            width: 200px;
            background-color: black;
            display: inline-block;
        }
    </style>
</head>
<body>
    <div id="box"></div>
</body>
</html>